(a) Field of the Invention
The present invention relates to a multiple frequency adaptive filter for removing undesired periodic and/or correlated interference signals contained in the same frequency band as a desired input signal.
(b) Description of Prior Art
In recent years, many different circuits or systems have been proposed for separating, enhancing, or cancelling noise, intersymbol interference, echo, or different signal frequencies affecting a desired signal. These prior art systems, particularly in the time domain, such as described in U.S. Pat. No. 4,052,559 to Paul et al and U.S. Pat. No. 4,238,746 to McCool et al, each teach the use of an adaptive transversal filter whose weight vector is governed by the Widrow-Hoff algorithm, a real time least mean square approximation to the Weiner-Hoff filter. Reference is herein made to Proceedings of the IEEE, Vol. 63, No. 12, December 1975, pp. 1692-1716, "Adaptive Noise Cancelling" by Widrow et al.
Further adaptive cancelling can also be performed in the frequency domain. Also, when the interference is simply a sinusoidal signal of known frequency, Widrow suggests the use of a pair of in-phase/in-quadrature signals to track the amplitude and phase of a sinusoidal interference in place of an adaptive transversal filter. Widrow's adaptive notch filters can be used in series or parallel combination for the case of multiple sinusoidal interferences whose frequencies are individually known.